1. Field of the Invention
The present invention relates to spread spectrum communication systems using PN coding techniques and, more particularly, to linear PN code searching to determine PN composite code phase.
2. Prior Art
Spread spectrum (SS) systems, which may be CDMA systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) PN-code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
In general, there are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the carrier frequency and Rc represents the PN-code chip rate, which in turn may be an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver sharing a common PN code or having its own designated PN-code. Although each receiver receives the entire frequency bandwidth, only the signal with the receiver's matching PN-code will appear intelligible; the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
As noted, the DSSS system PN-code sequence spreads the data signal over the available bandwidth such that the signal appears to be noise-like and random; but the signal is deterministic to a receiver applying the same PN-code to de-spread the signal. However, the receiver must also apply the same PN-code at the appropriate phase in order to de-spread the incoming signal, which explicitly implies synchronization between the receiver and transmitter. However, in group communication environments, such as a fleet battle-group where the battle-group composition changes regularly (daily or even hourly); or where the participants are engaged in a common training exercise, but geographically dispersed around the globe, typical synchronization techniques, such as resetting the start of the PN code for all the participants, is not practical. Moreover, communication interruptions due to resetting PN codes at an arbitrary time seam, such as days, weeks, months, and years, in a battle-group environment could have undesirable consequences. As used herein, a time seam occurs when a fleet of platforms begins its PN code from the beginning of a time event, such as the Global Positioning System (GPS) day in which the fleet assembles. The convention used by the fleet is to ignore subsequent GPS day boundaries once communication among the fleet has begun, meaning that the PN code shared among the fleet is not reset at subsequent GPS day boundaries.
In this manner, PN encoded communications can persist for two or three days. However, a platform that attempts to join the fleet and participate in fleet communications, subsequent to the beginning of the time event is confronted with a time and PN code phase ambiguity and will be unable to join fleet communications unless the ambiguities are resolved.
Some PN systems may be able to partially correlate the incoming composite PN-encoded signal with just one of the PN component codes, but at a reduced power level. Phase alignment with the other PN component codes may then be determined through information provided by the transmitting system. However, this approach has the disadvantage of bounding data rates by epochs of the partially phase aligned PN code.
Some systems may use three-component PN codes where acquisition is often achieved by searching (slipping or advancing) each PN component code for phase alignment with the composite PN-encoded signal one chip at a time; otherwise known as brute force searching. It will be appreciated that a disadvantage in brute force searching is that composite code phase is not preserved.
It will be appreciated that a disadvantage of advancing component PN codes, (i.e., independently withholding one or more clock cycles from the individual component PN code generators) is that phase information derived from the partially phase aligned composite PN-encoded signal is lost and must be regained.
In order to preserve knowledge of the partially aligned PN code phase, the entire composite code phase is slipped by one chip. Yet, this method is only valid for the first component code that is brute force searched.
With X-epoch synchronous data, θPN is preserved by composite code slipping, and θPN status (Δθ and TSI) may be recovered from the transmitted data stream since data may be recovered when there is partial phase alignment and bit synchronization (i.e., data edges are coincident with X-epochs).
With XY-epoch synchronous data, θPN is preserved through the X search phase but is likely to be lost during the brute force Y search phase. Yet, bit synchronization (data recovery) can only occur after X and Y phase alignment with the composite PN code.
It is therefore desirable to provide a method and system whereby information regarding composite PN code phase information is not lost when independently slipping or advancing component PN codes.